1. FIELD OF THE INVENTION
This invention relates to a circuit for correcting frequency response characteristics of the television signal and more particularly to a circuit which automatically controls band characteristics (frequency response characteristics) of the video intermediate frequency circuit so as to constantly keep the amplitude of a high frequency component of the picture signal correct, thereby making it possible to constantly display high quality pictures on the screen of the picture tube.
2. DESCRIPTION OF THE PRIOR ART
In color television receivers, in the event that the antenna is mismatched with the tuner or the tuner is erroneously tuned, the frequency response is degraded in the circuit ranging from the antenna to the tuner with the result that the amplitude of the picture signal or chrominance subcarrier contained in the received television signal becomes either excessive or insufficient. An insufficient amplitude of the picture signal, for example, is responsible for insufficient contrast and brightness. To cope with this problem, a typical television receiver is provided with an AGC (Automatic Gain Control) circuit which is adapted to keep the amplitude of the picture signal correct so that contrast and brightness are correctly maintained. Usually, the AGC circuit detects the amplitude of the horizontal synchronizing signal contained in the television signal and then controls the gains of the tuner and the intermediate frequency circuit in such a manner that the detected amplitude is kept invariable, thereby keeping the amplitude of the picture signal correct. However, since the AGC circuit is designed for control for keeping the amplitude of the relatively low frequency horizontal synchronizing signal constant, the amplitude of the low frequency component contained in the picture signal can be kept correct by the AGC circuit. However, the amplitude of high frequency component of the picture signal can not be kept correct by this approach.
An excessive or insufficient amplitude of the chrominance subcarrier, on the other hand, leads to an excessive or insufficient color saturation. A solution to this problem is the provision of an ACC (Automatic Color Control) circuit as disclosed in the U.S. Pat. No. 3,079,461 to E. O. Keizer, which ACC circuit detects the amplitude of color burst signal contained in the chrominance subcarrier and then performs an automatic control for keeping the detected color burst signal amplitude invariable to thereby keep the amplitude of the chrominance subcarrier correct.
As described above, the ordinary television receiver is incorporated with circuits for keeping correct amplitudes for the picture signal low frequency component and the chrominance subcarrier, respectively, whereas it usually lacks a circuit for keeping the amplitude of the high frequency component of the picture signal correct. Thus, in the event that the amplitude of the high frequency component of the picture signal is decreased owing to mismatching or erroneous tuning of the tuner, resolution of the picture to be displayed is decreased. Conversely, with an increased amplitude of the high frequency component, excessive overshooting or undershooting results. In addition, an excessive amplitude of the chrominance subcarrier is responsible for the generation of cross color and interference between the aural carrier and chrominance subcarrier which leads to beat frequencies, thus causing a beat pattern in fringes in the picture.
Needless to say, if the television receiver experienced complete assembling and adjustment, the aforementioned phenomena might not occur. Ideally complete adjustment, however, is rarely the case, and many television receivers go through insufficient adjustments before they are put in operation. Therefore, changes in the amplitudes of the picture signal high frequency component and the chrominance subcarrier is quite possible due to mismatching and erroneous tuning or mistuning.